Follow-up from "Namespace license checks for multiple assignees"

What does this MR do?

The following discussions from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1916 should be addressed:

• @DouweM started a discussion: (+3 comments)

  I think we can disable reassign outright if the issuable doesn't support multiple assignees, since in this case it's the same as assign. I don't think it's in CE either.

  In EE the /reassign code was move to a EE-specific file. This MR removes it completely from CE.

• @DouweM started a discussion: (+2 comments)

  Do we still need (CE) here?

  Removed the (CE) in the spec description.

• @DouweM started a discussion: (+1 comment)

  Since this spec is EE specific, can it go into a separate file?

  Same here, removed CE from the spec description.

• @DouweM started a discussion: (+1 comment)

  I think we should backport as much as possible in this file, and just know that in CE, allows_multiple_assignees? will always be false and point that out with some code comments.

  Removed /resassign from CE, see bullet above.

Anything the reviewer should double check

All the changes were already applied in EE by gitlab-org/gitlab-ee!1916. Including the removed code (for /reassign) being moved to a EE-specific file.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ee#2839.